1. Field of the Invention
The present invention relates to a semiconductor device which uses a metal gate electrode, and a method for manufacturing the same.
2. Description of the Related Art
To satisfy a desire for effectively thinning a gate insulating film in accordance with heightened performance and a heightened integration degree of a semiconductor device, it is essential to introduce technologies of a metal gate electrode and a high dielectric constant (high-k) gate insulating film in the future. To obtain adequate performance of an MIS transistor which uses a metal gate/high-k gate insulating film, an effective work function φeff of a metal gate material needs to be about 3.9 to 4.3 eV for an n-channel type MIS transistor and about 4.8 to 5.2 eV for a p-channel type MIS transistor (e.g., see JP-A 2007-173412 (KOKAI)).
However, a metal having a low work function suitable for the n-channel type MIS transistor is generally not stable in a heating step indispensable for a transistor formation process, and especially, the φeff of the metal on the high-k gate insulating film becomes a value close to a mid-gap (in the vicinity of 4.6 eV) after the formation of the transistor. Hence, it is difficult to realize a φeff value of about 3.9 to 4.3 eV suitable for the n-channel type MIS transistor. Therefore, a threshold voltage of the n-channel type MIS transistor cannot sufficiently be lowered. This leads to a serious problem in introducing the technologies of the metal gate electrode/high-k gate insulating film.
As understood for the above, in a case where the high-k gate insulating film/metal gate electrode is used, there occurs a problem that a low threshold voltage cannot be realized in the n-channel type MIS transistor. Therefore, there has been desired the realization of a semiconductor device of the n-channel type MIS transistor that has a reduced threshold voltage, and a manufacturing method which can form the device by using an easy process.